Wind energy systems are quickly becoming a popular form of power generation technology, and ongoing development is directed to providing wind-generated power to electrical grids. Power conversion systems are needed to adapt mechanical power generated by wind turbines to AC electric power in a form compatible with the grid. One type of conversion apparatus used in wind energy conversion systems (WECSs) is a double fed induction generator (DFIG) with a rotor driven by a turbine through a gearbox to supply power to a grid via stator connections. The DFIG rotor windings are connected to the grid via a back-to-back converter system having a rotor side converter connected between the rotor windings and a DC circuit, along with a grid side converter connected between the DC circuit and the grid. The system operates with the back-to-back converter drawing power from or supplying power to the rotor depending on the relationship of the rotor speed to the desired grid frequency. The system provides power to the grid via the stator windings with the rotor frequency often deviating from a nominal corresponding to the grid frequency. The back-to-back converter controls the rotor currents to adjust the active and reactive power fed to the grid from the stator independently of the rotor speed, and the DFIG generator is able to both import and export reactive power. This capability is advantageous in grid-tied systems as the DFIG system can be operated to support the grid during severe voltage disturbances (grid voltage sag conditions). This architecture also allows the generator to remain synchronized with the grid while the wind turbine speed changes, where variable speed wind turbines use the energy of the wind more efficiently than fixed speed turbines.
DFIG converters essentially operate in one of two modes, depending on the rotating speed of the rotor. For rotor speeds below the nominal rotational speed, some of the stator power is fed to the rotor via the converters, with the grid side converter stage operating as a rectifier to supply power to the intermediate circuit and the rotor side converter inverting the DC power to power the rotor windings. When the rotor speed is above the nominal value, rotor currents are used to power the intermediate circuit, and the grid side converter operates as an inverter to supply power to the grid.
The DFIG generator is typically constructed with significantly more rotor windings than stator windings such that the rotor currents are lower than the stator currents, allowing the use of a relatively small back-to-back converter, where the converter components are typically sized for operation within a certain rotor speed range. However, the DFIG rotor voltages are consequently higher than the stator and grid voltages, and thus the rotor side converter and intermediate circuit are particularly susceptible to voltage transients caused by grid disturbances. DFIGs therefore typically include a crowbar circuit connected to the rotor windings, which can activate a load to conduct rotor currents in the event of grid faults.
As WECSs become more prevalent, utility operators must ensure the reliability and efficiency of the power system, including compliance with grid connection codes applicable to distributed generators including wind power generators. One such requirement is the capability of WECSs to ride-through grid fault conditions without internal damage, while also providing some measure of remedial action to support the grid. Crowbar circuits are activated and the switches of the rotor side converter stage are opened upon detection of grid faults to protect the rotor and converter components from excessive voltage spikes.
However, the crowbar circuit needs to be deactivated while the grid fault continues, in order to allow the DFIG system to begin active regulation to prop up the grid to meet regulatory specifications for grid-tied operation. In this regard, restarting the rotor side converter allows provision of reactive current to the grid during the remainder of voltage sag type grid faults to help the grid to recover from the fault. However, voltage spikes caused by crowbar deactivation can prevent or hinder the ability to restart regulated operation of the back-to-back converter.
U.S. Pat. No. 7,164,562 to Virtanen, issued Jan. 16, 2007 attempts to solve this problem by using the rotor side converter switches to short-circuit the AC side of the converter to facilitate commutation of the crowbar protective switch so that normal operation can be resumed quickly after a failure situation. This approach, however, requires complicated converter control switching. EP 1 965 075 A1, published Sep. 3, 2008 describes a crowbar with multiple branches allowing control of rotor current with different strategies according to crowbar voltage, stator current, rotor current or DC-link voltage by sequential deactivation of the crowbar branches so that the rotor voltage is kept low enough that no current circulates towards DC-link intermediate circuit. This approach, however, requires extensive additional hardware and increases the cost and complexity of DFIG systems. Accordingly, there is a need for improved DFIG converters and techniques for wind energy systems by which energy derived from wind-driven machines can be converted to grid power while providing grid fault ride through capabilities with the ability to restart regulation for grid support after deactivation of a protective crowbar circuit.